1. Field of the Invention
The present invention is directed generally to pressure sensor systems.
2. Description of the Related Art
A miniature pressure sensor system can include a housing with an external diaphragm that directly contacts a sampled fluid, a sensor (enclosed by the housing) that is not in direct contact with the sampled fluid, and an internal intermediary fluid (contained within the housing) that contacts the external diaphragm and also contacts the sensor to allow the sensor to measure pressure of the sampled fluid. In order to accurately transfer pressure from the sampled fluid to the sensor, among other things, the external diaphragm must be as compliant with the sampled fluid as possible. Unfortunately, temperature changes can cause the intermediary fluid to expand or contract at a different rate than pertinent components of the housing thereby inducing pressure changes in the intermediary fluid unrelated to pressure status of the sampled fluid and consequently, affecting accuracy of pressure measurements of the sampled fluid.
Versions of microelectromechanical systems (MEMS) can determine pressure levels of a fluid being measured and can be especially useful for miniature pressure sensor systems due to their small size. MEMS pressure sensor dies typically have a MEMS diaphragm fabricated to be integrated in the MEMS die and are typically positioned to directly contact the fluid being measured. In some applications of miniature pressure sensor systems, a MEMS diaphragm may not be compatible with the particular environment and/or the fluid being measured such as intraluminal fluids found in biological environments like with blood flows in vascular structures.
In other cases the MEMS pressure sensor die may be part of other components sharing a common package. These situations can reduce the desirability of the MEMS diaphragm directly contacting the fluid being measured. In such cases the MEMS diaphragm may be enclosed inside of a housing having an external diaphragm and a internal intermediary fluid therebetween. The external fluid is able to contact the sampled fluid to be measured and the intermediary fluid transfers pressure experienced by the external diaphragm on to the MEMS diaphragm.
Selection of conventional diaphragms used in non-miniature pressure sensor systems for use as an external diaphragm in miniature pressure sensor systems involving MEMS can be problematic. For instance, highly-stable conventional pressure sensor systems for use in industrial applications are typically quite large (>15 mm in diameter), having large-diameter, thin metal corrugated diaphragms. Further, these industrial applications typically involve a wide range of pressures, which allows such large sized industrial diaphragms to impart a very small error in pressure measurement relative to the range of pressures being measured.
In applications involving miniature pressure sensor systems, such as with vascular structures, where the size of a pressure sensor system is relatively small and where measured pressures have small ranges and/or values, use of conventional diaphragms for an external diaphragm raise scaling issues when going from the larger scale applications to the smaller scale applications. For instance, in some smaller scale applications, such as biomedical applications in general and involving pulmonary artery pressure measurement in particular, a normal pressure range may be only from 0 Torr to 20 Torr on a gage basis.
In some of these cases (such as in an implanted medical device), an atmospheric reference pressure is not available, which requires pressure measurement using an absolute pressure range from 760 to 780 Torr absolute that leaves little room for error on a percentage basis and causes a high demand for accuracy and stability over both time and temperature. For example, typically, medical diagnosis under such conditions requires a measurement to within 1 Torr, or 0.13% on an absolute scale.
Some conventional miniature pressure sensor systems are filled with silicone or fluorosilicone gel coatings and use no diaphragm. The operational life spans of the coatings tend to be short due to the coatings so are not appropriate where sensor elements require long-term protection from corrosive body fluids. In addition, these conventional coatings expose sample intravascular blood to surfaces that are not fully hemocompatible, such as found with the silicone or fluorosilicone gel materials used for the coatings, which can lead to thrombus (clot) formation within a vascular structure hosting such a miniature pressure sensor system.
In addition, external diaphragm surface geometry is of importance in such cases when a miniature sensor system is to be placed in flowing fluids such as flowing blood. An undesirable geometry for fluid flow applications includes rough, non-hydrodynamic surfaces that can induce shearing motion within a blood flow thereby introducing a potential activation mechanism of a coagulation cascade, leading to thrombus (clot) formation.